Lithium secondary battery containing cathode materials having high energy density and organic/inorganic composite porous membrane

Abstract

Disclosed is a secondary battery including a cathode, an anode, a membrane and an electrolyte, wherein the cathode contains a mixture of a first cathode material defined herein and a second cathode material selected from the group consisting of a second-(a) cathode material defined herein and a second-(b) cathode material defined herein, and a combination thereof, wherein a mix ratio of the two cathode materials (first cathode material: second cathode material) is 50:50 to 90:10, and the membrane is an organic/inorganic composite porous membrane including (a) a polyolefin-based membrane substrate and (b) an active layer in which one or more areas selected from the group consisting of the surface of the substrate and a portion of pores of the substrate are coated with a mixture of inorganic particles and a binder polymer, wherein the active layer has a structure in which the inorganic particles are interconnected and fixed through a binder polymer and porous structures are formed by the interstitial volume between the inorganic particles.

The invention claimed is: 
     
       1. A secondary battery comprising a cathode, an anode, a membrane and an electrolyte,
 wherein the cathode comprises a mixture of a first cathode material defined below and a second cathode material selected from the group consisting of a second-(a) cathode material defined below and a second-(b) cathode material defined below, and a combination thereof, wherein a mix ratio of the two cathode materials (first cathode material: second cathode material) is 50:50 to 90:10, and 
 the membrane is an organic/inorganic composite porous membrane comprising (a) a polyolefin-based membrane substrate and (b) an active layer in which one or more areas selected from the group consisting of the surface of the substrate and a portion of pores present in the substrate are coated with a mixture of inorganic particles and a binder polymer, 
 wherein the active layer has a structure in which the inorganic particles are interconnected and fixed through the binder polymer, and porous structures are formed by the interstitial volume between the inorganic particles, 
 said first cathode material comprising a cathode material represented by Formula 1 below:
   Li x (Co y A m D z )O 1   (1)
 
 
 wherein 0.8≦x≦1.2, 0≦z≦0.3, 1.8≦t≦4.2, (0.8-m-z)≦y≦(2.2-m-z), 0≦m≦0.3, A is at least one selected from Mg and Ca, and D is at least one selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta; Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Au, Ag, Zn, Cd, Hg, B, Al, Ga, In, TI, C, Si, Ge, Sn, Pb, N, P, As, Sb and Bi; 
 said second-(a) cathode material comprising a cathode material represented by Foimula 2a below:
   Li x (Ni 1-a-b Mn a Co b ) y O 2   (2a)
 
 
 wherein 0.05≦a≦0.4, 0.1≦b≦0.4, 0.4<1-a-b<0.7, 0.95≦x≦1.05, 1.9<x+y<2.3; and 
 said second-(b) cathode material comprising a cathode material which contains a transition metal mixture of Ni, Mn and Co, has an average oxidation number of all transition metals except for lithium, larger than +3 and satisfies Equations 2b and 2c below:
   1.1 <m (Ni)/ m (Mn)<1.5  (2b)
 
   0.4 <m (Ni 2+ )/ m (Mn 4+ )<1  (2c)
 
 
 wherein m(Ni)/m(Mn) is a molar ratio of nickel to manganese and (Ni 2+ )/m(Mn 4+ ) is a molar ratio of Ni 2+  to Mn 4+ . 
 
     
     
       2. The secondary battery according to  claim 1 , wherein a mix ratio of the cathode materials is 50:50 to 70:30. 
     
     
       3. The secondary battery according to  claim 1 , wherein the first cathode material is LiCoO 2 . 
     
     
       4. The secondary battery according to  claim 1 , wherein, in the second-(a) cathode material, lithium ions are intercalated and deintercalated between mixed transition metal oxide layers (“MO layers”) and some Ni ions derived from MO layers are inserted into intercalation and deintercalation layers of the lithium ions (“reversible lithium layer”) to bond the MO layers. 
     
     
       5. The secondary battery according to  claim 4 , wherein Ni 2+  and Ni 3+  are present together in the MO layers and some Ni 2+  is inserted into the reversible lithium layer. 
     
     
       6. The secondary battery according to  claim 5 , wherein a molar fraction of Ni 2+  inserted into the reversible lithium layer in the second-(a) cathode material is 0.03 to 0.07, based on the total weight of transition metals of the second-(a) cathode material. 
     
     
       7. The secondary battery according to  claim 1 , wherein, in the second-(b) cathode material, m(Ni)/m(Mn) satisfies 1.2≦m(NO/m(Mn)≦1.4. 
     
     
       8. The secondary battery according to  claim 1 , wherein, in the second-(b) cathode material, the average oxidation number of transition metals other than lithium is higher than 3.0 and is lower than or equal to 3.5. 
     
     
       9. The secondary battery according to  claim 1 , wherein the nickel in the second-(b) cathode material is composed of nickel (a) present in an excessive amount, as compared to the manganese content, and nickel (b) present in an amount corresponding to the manganese content. 
     
     
       10. The secondary battery according to  claim 1 , wherein the nickel in the second-(b) cathode material has an average oxidation number higher than 2+. 
     
     
       11. The secondary battery according to  claim 9 , wherein the nickel (a) is Ni 3+ . 
     
     
       12. The secondary battery according to  claim 9 , wherein an average oxidation number of the nickel (b) is higher than 3.0 and is lower than or equal to 3.5. 
     
     
       13. The secondary battery according to  claim 9 , wherein the nickel (b) contains Ni 2+  and Ni 3+ . 
     
     
       14. The secondary battery according to  claim 9 , wherein Ni 3+  content ranges from 11 to 60% of the nickel (b) with the content corresponding to the content of manganese. 
     
     
       15. The secondary battery according to  claim 1 , wherein the content of Ni (Ni 2+ ) intercalated into lithium sites in the second-(b) cathode material is lower than 5 mol %. 
     
     
       16. The secondary battery according to  claim 1 , wherein the first cathode material is a monolithic particle and the second cathode material is an agglomerated particle composed of an agglomerate of micro particles. 
     
     
       17. The secondary battery according to  claim 1 , wherein the first cathode material has a D50 of 15 μm or more and the second cathode material has a D50 of 8 μm or less. 
     
     
       18. The secondary battery according to  claim 17 , wherein the first cathode material has a D50 of 20 to 30 μm and the second cathode material has a D50 of 4 to 7 μm. 
     
     
       19. The secondary battery according to  claim 18 , wherein 90% or more of the second cathode material is an agglomerate of micro particles having a size of 1 to 4 μm. 
     
     
       20. The secondary battery according to  claim 18 , wherein the inorganic particles are at least one selected from the group consisting of inorganic particles having a dielectric constant of 5 or more (a), inorganic particles having piezoelectricity (b) and inorganic particles capable of transferring lithium ions (c). 
     
     
       21. The secondary battery according to  claim 20 , wherein, in the piezoelectric inorganic particles (b), one side of the particles is positively charged and the other side thereof is negatively charged to cause a potential difference between both sides, when a predetermined pressure is applied. 
     
     
       22. The secondary battery according to  claim 20 , wherein the inorganic particles capable of transferring lithium ions (c) contain lithium elements, and do not store the lithium elements and transfer lithium ions. 
     
     
       23. The secondary battery according to  claim 20 , wherein the inorganic particles having a dielectric constant of 5 or more (a) are selected from the group consisting of SrTiO 3 , SnO 2 , CeO 2 , MgO, NiO, CaO, ZnO, ZrO 2 , Y 2 O 3 , Al 2 O 3 , TiO 2 , and SiC;
 the piezoelectric inorganic particles (b) are selected from the group consisting of BaTiO 3 , Pb(Zr,Ti)O 3  (PZT), and PB(Mg 3 Nb 2/3 )O 3 —PbTiO 3  (PMN-PT) and hafnia (HfO 2 ); and 
 the inorganic particles capable of transferring lithium ions (c) are selected from the group consisting of lithium phosphate (Li 3 PO 4 ), lithium titanium phosphate (Li x Ti y (PO 4 ) 3 , 0<x<2, 0<y<3), lithium aluminum titanium phosphate (Li,Al y Ti z (PO 4 ) 3 , 0<x<2, 0<y<1, 0<z<3), (LiAlTiP) x O y -based glasses (0<x<4, 0<y<13), lithium lanthanum titanate (Li x La y TiO 3 , 0<x<2, 0<y<3), lithium germanium thiophosphate (Li x Ge y P z S w , 0<x<4, 0<y<1, 0<z<1, 0<w<5), lithium nitride (Li x N y , 0<x<4, 0<y<2), SiS 2 -based glasses (Li x Si y S z , 0<x<3, 0<y<2, 0<z<4), and P 2 S 5 -based glasses (Li x P y S z , 0<x<3, 0<y<3, 0<z<7). 
 
     
     
       24. The secondary battery according to  claim 1 , wherein the binder polymer has a solubility parameter of 15 to 45 MPa 1/2 . 
     
     
       25. The secondary battery according to  claim 1 , wherein the binder polymer is selected from the group consisting of polyvinylidene fluorideco-hexafluoropropylene, polyvinylidene fluoride-cotrichloroethylene, polymethylmethacrylate, polyacrylonitrile, polyvinyl pyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyimide, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxylmethyl cellulose, polyvinyl alcohol and combinations thereof. 
     
     
       26. The secondary battery according to  claim 1 , wherein a material for the polyolefin-based membrane substrate is selected from the group consisting of high-density polyethylene, low-density polyethylene, linear low-density polyethylene, ultrahigh molecular weight polyethylene and polypropylene. 
     
     
       27. The secondary battery according to  claim 1 , wherein the secondary battery is a pouch battery in which an electrode assembly is sealed in a pouch-type case made of a laminate sheet including a resin layer and a metal layer.